LATE QUATERNARY STABLE OXYGEN ISOTOPE RECORD OF TANNER BASIN

The Site 1014 oxygen isotope record of G. bulloides exhibits the familiar climate-related curve for the interval since the penultimate glacial episode (<179 ka) (Fig. 4). Minimum Holocene ¹⁸O values of ~-0.5 are similar to G. bulloides ¹⁸O records in Santa Barbara Basin (Kennett, 1995). After these minimum values, the surface waters of the basin appear to cool as ¹⁸O values increase (~0.5). The maximum ¹⁸O values of the last glacial maximum recorded in the surface waters of Tanner Basin were ~2.5, which were lower than those recorded in Santa Barbara Basin (~3). If we assume no change in salinity, the 3 shift in ¹⁸O (-0.5-2.5) would suggest a ~7°C shift in the temperature of surface waters in Tanner Basin (assuming 1°C is equivalent to 0.23 ¹⁸O [Epstein et al., 1953]). The familiar apparent climate amelioration of MIS 3 can clearly be seen at Site 1014 as well as a concomitant increase in ¹⁸O variability.

Several differences can be observed between the present interglacial (Holocene) and the last interglacial (MIS 5). The planktonic ¹⁸O record is incomplete during the last interglacial because of several episodes of dissolution, which resulted in insufficient material for analysis (Fig. 5). The ¹⁸O values of this interval are also significantly higher than expected; in particular, the warmest interval of the last interglacial (the Eemian) was 0.8 more positive than the Holocene (Fig. 4). Such values, if correct, would indicate that after correcting for ice volume, last interglacial ¹⁸O values were similar to the glacial, suggesting no change in surface water temperature. It is clear, however, that the Eemian was at least as warm as, if not warmer than, the Holocene (CLIMAP members, 1984; Kennett, 1995). Hence, the Tanner Basin record during this interval was affected by other processes. Our preferred explanation involves the preferential dissolution of thin-shelled specimens, which generally grow at shallower depths and warmer temperatures, biasing the remaining specimens toward higher ¹⁸O (and thus cooler temperatures). This conjecture is supported by independent methods of estimating SST such as U^k₃₇, which suggest that temperatures during marine isotope Substages 5a and 5c were at least as warm as the Holocene and that the Eemian was 3° warmer (Yamamoto et al., 1998). Another explanation involves the increased abundance of the small morphotype of Gephyrocapsa species, dominant in upwelling regions during warm interglacial episodes. This suggests that upwelling was more intense during these intervals (Yamamoto et al., 1998). Increased upwelling causing cooler SSTs may not be reflected in the U^k₃₇ temperatures since it has been shown that U^k₃₇ values also vary with nutrient availability (Epstein et al., 1998). The strongest evidence against the lowering of sea-surface temperatures by upwelling is the lack of evidence for cool sea-surface temperatures at other sites in the region during this time interval.

Changes in planktonic foraminiferal ¹⁸O at Site 1014 indicate large SST shifts (~7°C) between the last glacial maximum (MIS 2) and the Holocene (MIS 1). Such large shifts are suggested by earlier ¹⁸O investigations of the late Quaternary in Tanner Basin, although they were of much lower resolution. Mortyn et al. (1996) found Holocene ¹⁸O values of ~0.5 to -0.5 and glacial maximum ¹⁸O values close to 2.5 of G. bulloides. Based on these differences and planktonic foraminiferal assemblages (modern analog technique-derived temperatures), it was theorized that SSTs in Southern California were 7° to 8°C cooler during the last glacial maximum (MIS 2). In contrast, Kahn et al. (1981) suggested slightly less glacial-interglacial SST change of ~5°C based on a ¹⁸O shift from Holocene values of ~0.5 and last glacial maximum values of ~2.0 in the planktonic foraminifer Globigerina quinqueloba. Temperature estimates based on organic geochemical signals (U^k₃₇) produced from Site 1014 also suggest an ~5° temperature change between MIS 3 and the Holocene (Yamamoto et al., 1998).

Although changes in the benthic ¹⁸O record are similar to those of the glacial deep-sea average (Martinson et al., 1987), the differences appear important. During Termination I, the total shift in ¹⁸O between the glacial maximum and the Holocene is ~2 (Fig. 6). It is now well established that changes in oceanic ¹⁸O composition during this interval were ~1.1 because of deglaciation of the Earth's cyrosphere (Shackleton and Opdyke, 1976). Thus, 0.9 of the shift in benthic ¹⁸O must have resulted from temperature and salinity change. Assuming no salinity change and that 1°C is equivalent to 0.23 (Epstein et al., 1953), a temperature change of ~4°C would be indicated. Such a large temperature variation would imply a significant shift in the source of intermediate waters in the North Pacific as previously suggested (Kennett and Ingram, 1995; Behl and Kennett, 1996). However, this intermediate water temperature change in the Tanner Basin (~1165 m in depth) appears to be too large since modern bottom temperatures are already at ~3.8°C (Emery, 1954). Near-freezing waters would have resulted, which is unlikely in a shallow water mass at middle latitudes. Clearly, a salinity change must also have contributed to the benthic ¹⁸O shift at Site 1014. As a 1 salinity shift is equivalent to 0.5 ¹⁸O (Craig and Gordon, 1965), it is possible that a significant salinity increase accompanied a cooler intermediate water source. Bottom temperatures could have been ~2°C because glacial North Pacific Intermediate Water may have had a higher component of Pacific Arctic water during the last glacial maximum (Keigwin, 1998). A more reasonable 2°C decrease in temperature would leave 0.45 of the ¹⁸O to be accounted for by a salinity increase of ~0.9.

The benthic oxygen isotope record also suggests that there are some differences in the response of benthic ¹⁸O to global warming during the Eemian and the Holocene. In particular, the early Holocene ¹⁸O values are ~0.2 lower than Eemian values (Fig. 6), implying that bottom waters were warmer or less saline during the early Holocene than during the last interglacial. Furthermore, the duration of the benthic ¹⁸O decrease at Termination I was much longer (~10 k.y.) than during Termination II (~5 k.y.) (Fig. 6).